The present invention relates to a method, a security module and a system according to the preamble of the independent claims.
In particular, the present invention relates to a method for producing an output bit stream for a first signal of a first carrier frequency by a security module. The security module receives an input signal comprising the first signal and a second signal of a second carrier frequency. The input signal is demodulated with the aid of a first nonlinear component, wherein the first nonlinear component outputs a first baseband signal. A first bit stream is generated from the first baseband signal. The first bit stream is fed to an output logic. A mixed signal is formed from the first signal and the second signal, wherein the mixed signal comprises the first signal at the first carrier frequency, the second signal at the second carrier frequency and a mixed product at an intermediate frequency.
Further, the present invention relates to a security module comprising a receiving unit for receiving a first signal of a first carrier frequency and a second signal of a second carrier frequency. The security module comprises a first nonlinear component and a level-value adjusting unit. The first nonlinear component is adapted to form a first baseband signal from the first signal and the second signal, wherein the level-value adjusting unit generates a first bit stream from the first baseband signal.
Different methods are known for receiving input signals and demodulating signals, in particular by a contactless security module. A receiving unit receives a first signal from a transmitting-/receiving device. The received first signal is usually modified by means of an impedance converter, in particular an apparatus and/or unit for multiplying a voltage. Via the impedance converter, an amplitude, in particular a voltage amplitude, of the received first signal is increased. When the first signal is an amplitude-modulated signal, the first signal is demodulated with the aid of the impedance converter comprising at least one nonlinear component. The impedance converter is usually a voltage multiplier.
To generate an output bit stream from the first signal, said first signal is processed by means of a level-value adjusting unit. The level-value adjusting unit produces a bit stream of the first signal from the demodulated first signal. The level-value adjusting unit usually comprises the function of a comparator, with which the demodulated first signal is converted into a binary signal. The demodulated first signal is further smoothed in a second current path by means of a smoothing unit and a reference value for the comparator is formed therefrom. The reference value, which is preferably lower than the peak amplitude of the demodulated first signal at the input of the level-value adjusting unit, is considered as a reference value vis-à-vis the demodulated first signal for generating the first output bit stream. The level-value adjusting unit thus evaluates the demodulated first signal with respect to the reference value.
Due to the constant rise of wireless communication, the number of signals which are modulated at different carrier frequencies is increasing. The contactless security module frequently receives further signals, but at least one second signal, in addition to the first signal. Particularly when the second signal has a second carrier frequency that is close to the first carrier frequency of the first signal, and thus the first carrier frequency and the second carrier frequency are in the same reception range of the security module, and in addition the second signal has an amplitude approximately of the order of the first signal or higher, the two carrier frequencies of the first signal and of the second signal influence each other in such a fashion that the security module cannot generate from the input signal a unique output bit stream relating to the first signal. The security module only generates a disturbance value. Communication between the transmitting-/receiving device and the contactless security module is no longer ensured.
For example, the frequency ranges of GSM and UHF RFID are not only immediately adjacent, but overlap each other partially. For example, a UHF-RFID reading device transmits in the frequency range of 865 MHz and a GSM mobile phone transmits in the frequency range of 880-915 MHz. Due to a simple construction type, a UHF RFID transponder is ready to receive over a relatively broad band. The reception range of the UHF RFID transponder is usually limited only by the antenna. The UHF RFID transponder can receive both UHF RFID signals and GSM signals of an adjacent mobile phone. Both signals superimpose. The UHF RFID transponder of the state of the art cannot separate and decode the UHF RFID signals. Communication between the UHF RFID transponder and a UHF-RFID transmitting-/receiving device is not possible, in particular as soon as the amplitude of the GSM signal exceeds a threshold amplitude in relation to the UHF RFID signal.